Process of improving the hand of polyethylene terephthalate fabric by heat shrinking and hydrolyzing the fabric



United States atent Paul Knapp, Wilmington, Del., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware N Drawing. Application February 5, 1954, Serial No. 408,604

5 Claims. (Cl. 8-115.5)

This invention relates to fabric treatment, particularly treatment of polyester fabrics to improve their handle.

Although difiicult to define objectively, a good handle is a prime requisite for commercial acceptance of textile fabrics. The difiiculty of objective definition of desirable handle is no great handicap for persons skilled in the textile arts, however, who can judge the extent to which a fabric has this characteristic by feeling it only briefly. A desirable handle to a fabric may be characterized by the adjectives lofty and luxurious or by other similarly subjective terms.

Fabrics made of recently developed synthetic fibers often are unacceptable to the touch. Prominent among compositions for these fibers are products of polymerization of glycols with terephthalic acid or its derivatives. The present invention is directed toward improving the handle of fabrics containing such fibers, for which the most familiar composition doubtless is polyethylene terephthalate.

Accordingly, an object of this invention is improvement in the handle of fabrics containing polyester fibers. Another object is treatment of polyester fabrics by first relaxing them and then subjecting them to appreciable hydrolysis. Other objects will be apparent from the following description of the invention.

In general, the objects of this invention are attained by first subjecting polyester fabric in relaxed condtion to dry heat and then treating the fabric with aqueous hydroxide of an alkali metal or salt thereof derived from a weak acid. Fabrics of ordinary construction shrink from several percent to ten percent, or so, in the warp direction and in the filling direction while undergoing this first step of the process. During the second or hydrolytic step the fabric loses an appreciable fraction of its original weight, perhaps one tenth to one fifth, but the outstanding effect of the complete process is the great resulting improvement in the handle of the fabric.

The first step of the process of the invention is satisfied by a dry heating of the subject fabric (which may be moist to start with) at high temperature while free to shrink. Although apparently interrelated, time and a temperature conditions for this step lie within fairly welldefined ranges. For best results, the temperature should be at least about 200 C. but should not exceed 230 C. Lower temperatures require prohibitively long heating times or completely fail to produce the desired improvement, and higher temperatures contribute an undesirably papery characteristic to the ultimate handle. The upper part of the above temperature range is most satisfactory for staple fabrics, the lower part for fabrics made of continuous filaments. Conventional textile equipment is capable of providing the desired conditions. As rate of heat transfer to the fabric is a factor in the requisite heating time, the fabric should remain in a pin tenter, for example, longer than in contact with a heated roll or other metal surface in performing this step. The time of heating is generally on the order of minutes; usually several minutes will sufiice, and rapid transfer of heat may reduce the time to a few seconds. During the dryheat treatment the fabric underoges a shrinkage of at least 3% warpwise and 3% fillingwise. Upon completion of this first step the fabric is stiff and boardy, giving little or no indication of the benefit conferred, which remains latent until performance of the next step of the process.

The second step is a treatment of the fabric with aqueous hydroxide of an alkali metal or salt thereof derived from a weak acid. It is convenient to immerse the fabric in a bath containing the desired compound in solution. For most satisfactory results, the aqueous bath will be at or near normal boiling temperature, atmospheric pressure being completely suitable. Perhaps the hydroxide most likely to be used, because of its cheapness, is sodium hydroxide. Potassium hydroxide is also readily available, although somewhat more expensive, and the hydroxides of the other alkali metals are less readily available and consequently more expensive, though otherwise suitable. Concentration of hydroxide in the bath need be only a few percent based upon the bath weight, which itself ordinarily will be a goodly number of times the weight of the subject fabric. When a salt is used instead, a higher concentration is often desirable. Salts of strong acids (sulfates, halogenides, etc.) are unsuitable at any concentration; for the purposes of this invention a salt is suitable if it produces a pH of at least 12 at concentration of 0.1 N. Useful salts include phosphates, silicates, sulfides, and sulfites, among others. Mixtures of hydroxides or of salts or of hydroxide and salt may prove especially beneficial in application of this process to particular fabrics. equipment suitable for accomplishing this step, as well as for performing subsequent scouring and rinsing, are the well-known jig and beck, the former being perhaps more appropriate for fabrics made of continuous filaments and the latter for staple fabrics.

Scouring and rinsing are desirable to clear the fabric of any residual hydrolyzed fiber and hydrolyzing medium left over from the second step of the process. The scouring solution may contain derivatives of higher fatty acids or other customary detergents. Upon subsequent drying at ordinary temperature (usually not more than C.) the fabric exhibits a most desirable handle. This improvement may be attributed at least in part to observable decrease in fiexural rigidity and increase in torsional elasticity, good handle being apparently dependent upon a subtle combination of these characteristics. The undesirable initial feel of the fabric is completely absent, having been superseded by a greatly preferable handle. No such improvement is afforded by known hot aqueous or aqueous-inorganic. treatments.

The following examples illustrate the practice of this invention in some detail. The improvement in handle of the fabric when treatedaccording to this invention may be judged from the data shown in the tables following the examples. The comparison extends not only to the original untreated fabric (merely scoured) but also to otherwise identical samples of fabric that have been exposed to a caustic process alone. Flexural rigidity was measured by the hanging heart method originated by Pierce (v. Journal of the Textile Institute 21, T377, 1930) and performed as described by Hoffman and Beste in Textile Research Journal XXI, No. 2, 66 (February, 1951); while torsional elasticity was determined by the following procedure, which was developed recently in an effort to establish a test correlating well with the observed reaction of fabric as it springs back" after being crumpled by hand.

A fabric strip measuring 4 inches by about 2 feet is formed at one end into a loop about /2 inch in diameter by sewing the end back upon the body of the strip. A

Examples of conventional arsnaae cylindrical half-inch aluminum rod 12 inches long (weight 108 grams) is centered in the loop and the other end of the strip clamped so that the top edge of the rod hangs 21 inches below the clamp. The rod is rotated manually through a horizontal angle of 1440 (4 complete turns), twisting the fabric strip. The rod is released to rotate in the opposite direction, and the value reported is equal to the number of degrees by which the maximum free counter-rotation exceeds 2160 (6 complete turns).

Example I Polyethylene terephthalate is made into continuousfilament yarn at a draw ratio of 3.5X. This yarn, having 34 filaments and a total denier of 70, is woven into a plain taifeta of 138 X 96 construction, weighing slightly over 2 ounces per square yard. This fabric is taken from the loom and heated in relaxed condition (free to shrink) in a pin tenter at a temperature of about 210 C. for 2 minutes. The fabric shrinks about 8% Warpwise and 9% fillingwise and becomes stiff and boardy. then is immersed for 15 minutes in a boiling caustic solution containing by weight of sodium hydroxide, the weight of the solution being about 40 times the Weight of the fabric. The fabric is removed and rinsed with warm water for 5 minutes, treated with 1% acetic acid for 5 minutes, again rinsed with warm water for 5 minutes, and finally dried at about 95 C. Weighing reveals that the fabric is 14% lighter than originally. The fabric now has a much more desirable handle than the original fabric.

TABLE 1 Fabric sample Flexural rigidity Torsional elasticity Example II A 2X2 twill suiting of 70 x 66 construction weighing approximately 6.6 oz. per square yard is woven from 30/2s yarn (cotton count) made of polyethylene terephthalate staple. The individual fibers which have been subjected to a draw of 4X, are of 3 denier and 2 /2 inches long.' This fabric is taken from the loom. and heated in relaxed condition (free to shrink) in a pin tentcr. The temperature is 229 C. and the time of heating 2 minutes. Warpwise shrinkage is 6%, fillingwise shrinkage 8%. At the end of this step the fabric is stiff and boardy. The heated fabric is then immersed in a boiling caustic solution (bath ratio about 40:1) containing 5% by weight of sodium hydroxide. The caustic treatment is continued for 15 minutes, whereupon the fabric is removed and rinsed with warm water for 5 minutes, treated with 1% acetic acid for 5 minutes, again rinsed with warm water for 5 minutes, and finally dried. The fabric has undergone a Weight loss of 16%. The treated fabric exhibits a much more desirable handle than the original fabric.

TABLE 2 Fabric sample Torsional elasticity The fabric individually, low flexural rigidity is a measure of good drape, and high torsional elasticity is commensurate with liveliness; the two together combine to constitute an index of desirable handle. The good values shown above for both these characteristics in fabric treated according to this invention indicate clearly the benefit conferred thereby, regardless of the di'fiiculty of. defining the handle function completely in terms of these or other variables. The untreated controls in the above tables were fabrics identical with the starting materials of the examples, only having been scoured in conventional manner and framed to dry after completion of the weaving operation. The caustic-treated controls were subjected to treatment like that of the second step in the examples; however, it is notable that in the absence of the first step (as with these control fabrics) the period of caustic treatment must be greatly extended over that of the examples to obtain comparable weight loss.

The desirable handle conferred according to the present invention upon fabrics containing polyester fibers may be characterized by other subjective terms and by other physical tests than those mentioned above; however, the result of the treatment is unmistakable. The fabric treated may be a blend containing yarns or fibers made of other components not deleteriously affected by the treatment, such as rayon but not wool. Fabrics containing substantial amounts of polyester staple or continuous filaments with accompanying unpleasant handle are considered to be polyester fabrics for the purposes of this invention. The advantages of practicing the present invention upon polyster fabrics generally are readily apparent.

What is claimed:

1. In the process for finishing a polyethylene terephthalate fabric, the improvement comprising first subjecting the fabric, while free to shrink, to dry heat at a temperature between about 200 C. and about 230 C. until the fabric has shrunk at least 3% Warpwise and at least 3% fillingwise, and then immersing the fabric in an aqueous solution at about boiling temperature until the fabric has lost between about 10% and about 20% in weight, said solution containing about 5% of a compound selected from the group consisting of alkali-metal salts of weak acids characterized by a pH of at least 12 in aqueous solution at a concentration of 0.1 N' and alkalimetal hydroxides, and then drying the fabric, said fabric being free from wool and other components deleteriously afiected by the aforesaid treatment.

2. The process of claim 1 in which the fabric is subjected to dry heat for at least two minutes.

3. The process of claim 1 in which the aqueous solution contains an alkali-metal hydroxide.

4. The process of claim 1 in which'the fabric is polyethylene terephthalate.

5. The process of claim 3 in which the alkali-metal hydroxide is sodium hydroxide.

References Cited-in the file of this patent FOREIGN PATENTS 1 610,171 Great Britain Oct. 12, 1948' 610,184 Great Britain Oct. 12, 1948 652,948 Great Britain May2, 1951 OTHER REFERENCES British Rayon and Silk Journal, June 1953, pg. 59. Marvin: Journal of the Society of Dyers and Colorists, January 1954, pp. 1621. 

1. IN THE PROCESS FOR FINISHING A POLYETHYLENE TEREPHTHALATE FABRIC, THE IMPROVEMENT COMPRISING FIRST SUBJECTING THE FABRIC, WHILE FREE TO SHRINK, TO DRY HEAT AT A TEMPERATURE BETWEEN ABOUT 200*C. AND ABOUT 230*C. UNTIL THE FABRIC HAS SHRUNK AT LEAST 3% WARPWISE AND AT LEAST 3% FILLINGWISE, AND THEN IMMERSING THE FABRIC IN AN AQUEOUS SOLUTION AT ABOUT BOILING TEMPERATURE UNTIL THE FABRIC HAS LOST BETWEEN ABOUT 10% AND ABOUT 20% IN WEIGHT, AND SOLUTION CONTAINING ABOUT 5% OF A COMPOUND SELECTED FROM THE GROUP CONSISTING OF ALKALI-METAL SALTS OF WEAK ACIDS CHARACTERIZED BY A PH OF AT LEAST 12 IN AQUEOUS SOLUTION AT A CONCENTRATION OF 0.1 N AND ALKALIMETAL HYDROXIDES, AND THEN DRYING THE FABRIC, SAID FABRIC BEING FREE FROM WOOL AND OTHER COMPONENTS DELETERIOUSLY AFFECTED BY THE AFORESAID TREATMENT. 